[NeIII]/[OII] as an Ionization Parameter Diagnostic in Star-Forming Galaxies

TL;DR

This paper introduces the Ne3O2 ratio as a new, reliable diagnostic for measuring the ionization parameter in star-forming galaxies, especially useful at high redshifts due to its insensitivity to reddening.

Contribution

It provides the first calibration of the Ne3O2 ratio as an ionization parameter diagnostic, based on advanced photoionization models, and demonstrates its effectiveness compared to existing methods.

Findings

Ne3O2 correlates tightly with O3O2 in star-forming galaxies.

Ne3O2 is less affected by reddening, making it suitable for high-redshift studies.

The calibration extends the tools available for analyzing galaxy ionization at z~1.6 to z~5.2.

Abstract

We present our parameterizations of the log([NeIII]3869/[OII]3727) (Ne3O2) and log([OIII]5007/[OII]3727) ratios as comparable and effective diagnostics of ionization parameter in star-forming galaxies. Our calibrations are based on the most recent generations of the Starburst99/Mappings III photoionization models, which extend up to the extremely high values of ionization parameter found in high-redshift galaxies. While similar calibrations have been presented previously for O3O2, this is the first such calibration of Ne3O2. We illustrate the tight correlation between these two ratios for star-forming galaxies and discuss the underlying physics that dictates their very similar evolution. Based on this work, we propose the Ne3O2 ratio as a new and useful diagnostic of ionization parameter for star-forming galaxies. Given the Ne3O2 ratio's relative insensitivity to reddening, this ratio is particularly valuable for use with galaxies that have uncertain amounts of extinction. The short wavelengths of the Ne3O2 ratio can also be applied out to very high redshifts, extending studies of galaxies' ionization parameters out to z ~ 1.6 with optical spectroscopy and z ~ 5.2 with ground-based near-infrared spectra.